1. The Field of the Invention
The present invention is related to surface cleaning and preparation of aluminum or other metal substrates in order to provide improved bonding to those substrates. More particularly, the present invention is related to cleaning such metal substrates first with an aqueous silicate solution, followed by treatment with an aqueous silane rinse.
2. Technical Background
In constructing various structures from metals it is important to have the capability of bonding to metal surfaces. This includes bonding metal surfaces to other metal surfaces, as well as bonding non-metal materials to metal surfaces. In many applications it is possible to use simple mechanical bonding mechanisms, such as bolts, screws, or rivets. In other applications, concerns over the added weight of mechanical fasteners make the use of adhesive more viable. Various adhesives are known and commonly used in the art of bonding metals together or bonding non-metal materials to metals. For example, various epoxy-based adhesives are widely used for these applications.
When metals are bonded using an adhesive it is generally important to provide the strongest possible bond. In the past it was difficult to assure a strong bond when using adhesive. For example, processing conditions during bond fabrication often cause dramatic reductions in bond strength. This is particularly true when bonding to metals such as aluminum. Bonding to aluminum and aluminum alloys has presented a special challenge.
Aluminum and aluminum alloys are considered difficult metals to bond to because of the propensity of aluminum surfaces to form a weak hydrated surface layer of aluminum oxide. The surface morphology and hence, adhesive bond durability, is dependent upon the type of surface treatment received prior to bonding. This is especially true for aluminum adherends because of the potential to form mechanically weak hydrated forms of aluminum oxide.
The oxide generally found on aluminum is the trivalent aluminum oxide (Al.sub.2 O.sub.3). This oxide is stable for bonding. However, with time and humidity, this oxide can become hydrated to form a less stable and mechanically weak layer of AlOOH (boehmite) and alumina trihydroxide (bayerite). The thickness of this layer can range from about 100 .ANG. to about 1500 .ANG., depending on the environment. If an adhesive is applied over this film, a weak boundary layer is incorporated into the bond system. The presence of this weak boundary layer results in decreased bond strength, low fracture toughness, and poor bond durability over time. Similarly, it is often a problem to provide bonds which are of sufficient initial strength.
A widely used treatment for aluminum surfaces involves vapor degreasing and grit blasting. However, these methods do not prevent further growth of boehmite and bayerite layers on the aluminum surface. Therefore, the bondline properties are very sensitive to the relative humidity and process times from surface preparation to bonding. Moisture can also penetrate the bondline during aging, resulting in degradation of bondline properties and interfacial failures in the boehmite/bayerite layer.
Available cleaning solvents used in this process have become more restrictive because of environmental regulation on chemical waste disposal. Thus, the combination of process sensitivity, marginal bond durability, and environmental constraints, raises concerns over continued use of traditional surface treatment processes.
There are limited alternative methods of aluminum surface treatment and preparation. Some of these other methods of aluminum surface preparation involve formation of stable, moisture-resistant oxide layers. These methods include sulfuric, chromic, and phosphoric acid anodization. These electrolytic processes inhibit the further growth of hydroxide layers and enhance initial bond strength and bond durability. In addition, the phosphoric acid process produces a honeycomb surface which is believed to enhance bond strength through mechanical interlocking.
These processes, however, generally consist of a complex series of treatments including degreasing, alkaline cleaning, acid etching, acid anodization, and in some instances, a post treatment process including primers and coatings. These processes use heavy metals, acids, caustics and other hazardous and toxic chemicals which pose handling and disposal problems. Clearly, environmental constraints limit the usefulness of these traditional surface treatment processes.
Accordingly, what is needed in the art are effective and efficient methods of surface preparation and treatment of aluminum, and other metals, in order to provide stable adhesive bonding to metal substrates. In that regard it would be a significant advancement in the art to provide methods of surface treatment and preparation which were relatively simple, and which used readily available materials. It would be a related advancement in the art to provide such methods which employed materials that did not present a significant environmental hazard.
It would be a further advancement in the art to provide methods of surface treatment and preparation which provided increased bond strength. It would also be an advancement to provide such methods which resulted in stable bonds which did not significantly degrade over time. It would be another advancement in the art to provide such methods which resulted in bonds which were unlikely to fail.
Such methods are disclosed and claimed herein.